The invention relates to a temperature regulator, particularly for an air-mass flow meter.
Air-mass flow meters are used particularly for determining flow of inlet air to an internal combustion engine. Heating element anemometers, also denoted thermal air-mass flow meters, are conventionally used for the design of air-mass flow meters. Two sensors or detectors are provided, one of which detects the temperature of the aspirated air and the other of which is heated to a specific temperature. The detectors are disposed in different branches of a bridge circuit. The detector that is heated and serves as the measurement sensor is cooled by the air flow depending on its velocity and temperature. The additional energy input required for bridge balance is thus a measure of the air mass flow that has passed through the detector. When there are large deviations from a reference temperature, however, measurement errors arise, due to the change in thermal conductivity of the air with respect to temperature.
In order to compensate for this measurement error, it is known, for example, to use a third sensor, as disclosed in DE 37 22 385 A1. Thereby, the temperature-dependent measurement error is corrected by suitable setting of the basic resistance value and its temperature coefficient.
A temperature regulation circuit for achieving a constant temperature of a heating resistance in a thermal air-mass flow meter is disclosed in DE 41 30 513 C2. Also, a high-resistance measuring resistor detecting the ambient temperature and a heating resistor are arranged in two different branches of a bridge circuit. By a suitable selection of resistance ratios of one resistor to the other, the heating resistor heats up in order to balance the bridge. The bridge branch with the temperature resistor essentially has a higher resistance than the bridge branch with the heating resistor, so that the intrinsic heat of the temperature resistor is kept negligibly small in order to exclude thermal measuring errors. It is a disadvantage that the high-resistance temperature resistor used for this purpose has a large space requirement for the sensor.
An object of the invention is to provide a temperature regulator, which is reduced in its size while maintaining the bridge conditions.
The invention is based on the concept that in spite of the condition that one bridge branch is configured with higher resistance compared to the other branch, in order to reduce the intrinsic resistance values, the size can also be kept small.
This is achieved by creating a voltage reduction in both bridge branches by additional means. With a smaller supply voltage, the resistance value of the temperature sensor can consequently be reduced, whereby the dimensions of the temperature sensor are also reduced. The same reduction in voltage is produced in the other bridge branch containing the heating resistor in order to keep the bridge in equilibrium. Subsequently, a post-regulation of the heating resistance is produced in a conventional manner by an operational amplifier upon an imbalance in the bridge caused by an increased temperature in the resistance of the temperature sensor. For this purpose, the resistance of the temperature sensor and the heating resistance have the same type of resistance values that change in a temperature-dependent manner. The central taps of the two bridge branches that form the null branch of the bridge are joined to the positive and negative inputs of the operational amplifier.
By the selection of smaller resistance values and the thus associated smaller dimensions of the resistor of the temperature sensor and the heating resistor, these components may also be connected to a common carrier, so that the flow direction can also be determined in a simple way.